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成果報告書詳細
管理番号20190000000235
タイトル*平成30年度中間年報 エネルギー・環境新技術先導プログラム/未踏チャレンジ2050/超高効率・高出力モーターに資する世界最強磁石の開発
公開日2019/5/14
報告書年度2018 - 2018
委託先名国立大学法人秋田大学
プロジェクト番号P14004
部署名イノベーション推進部
和文要約スパッタ成膜されたFeCo-XY(X=V,Y=N)薄膜の結晶構造解析結果から、FeCo薄膜の場合には、膜厚の増加に伴いbct構造は徐々に不安定化して5 nm程度でほぼ完全に元のbcc構造へと変態した。それに対して今回のFeCoにVNを同時添加した試料では、膜厚が20 nmにおいてもbct構造が維持されており、さらにN添加量に依存して結晶構造はbcc構造からbct構造を経てさらにはfcc構造までほぼ連続的に変態した。
一軸磁気異方性定数Kuは、軸比c/aが1.25付近の試料で最大値を示しており、理論計算結果と定性的に一致した。
第三元素無添加のFeCo薄膜の場合には膜厚が5 nm程度でほぼ完全にbcc構造(c/a=1.0)へと変態したのに対し、今回のFeCo-VNでは膜厚が100 nmにおいてもbct構造が維持され、さらに膜厚100 nmでN添加量を9.8 at%まで増大させることで、結晶構造はbct構造からfcc構造まで変態した。以上より、FeCo-VN薄膜では、添加するN量をコントロールすることで、結晶構造をbccからfccまで比較的自由に制御可能であると言える。このことから、将来的にはFeCoへのVN添加は、バルク形態でもbct構造を安定化する可能性が高いと考える。
英文要約The continuously increasing power consumption of motors utilised in electrical vehicles and air conditioners, and magnetic devices inside hard disk drives and random access memory chips has become a serious issue. Permanent magnets represent very important components of these applications. They are typically used in the bulk form in motors, actuators, and flux sources, and as thin films in storage and spintronic devices. Hence, enhancing the performance of permanent magnets represents the simplest and most efficient method for reducing their power consumption. The energy utilised by these magnets depends on the magnitudes of the saturation magnetization (Ms) and uniaxial magnetic anisotropy (Ku) of magnetic materials, whose high thermal stability can be achieved at high values of the Curie temperature (Tc).

FeCo with the body-centred cubic (bcc) structure is a well-known magnetic material characterised by the highest Ms and very high Tc values among various transition metal alloys. Although FeCo-based materials exhibit strong magnetic properties, their low Ku magnitudes make them unsuitable for the fabrication of permanent magnets. However, if the Ku of FeCo could be increased to a sufficiently high value, one of the strongest permanent magnets would be obtained.

In the equilibrium phase diagram constructed for FeCo, the fcc phase is stable at temperatures higher than 1258 K. Its transformation to the bcc phase occurs at lower temperatures, leading to the formation of the CsCl-type (B2) ordered bcc structure at temperatures below 1003 K without producing a bct intermediate. However, after considering the Bain transformation, two methods can be used to stabilise the bct structure: (A) applying a uniaxial stress to the FeCo lattice via epitaxial effects and (B) adding a third element to the FeCo structure.

Various experimental studies based on method A have been performed to investigate the magnetic anisotropy of FeCo by epitaxially growing it on several Rh, Pd, Ir, Pt, or CuAu buffer layers. Several experimental studies based on method B have been conducted as well. The addition of certain third elements is expected to generate a macroscopic tetragonal distortion in the FeCo lattice leading to the relaxation of the local stress in their vicinity. B, C, and N represent typical interstitial elements that are widely used for the manufacture of tetragonal Fe-based alloys.

In our previous study, we focused on the use of vanadium as the third additive element because of its ability to form a bcc solid solution around Fe50Co50 clusters, which was subsequently transformed into the fcc phase with an increase in the V content. A stabilisation of the bct phase was expected to occur at the boundary between the bcc and fcc phases. Although V addition was expected to stabilise the bct phase, the examined FeCoV and FeCoVC films were unable to achieve c/a = 1.2.

In this study, the effect of the VN addition to FeCo films on their tetragonal deformation and magnetic properties was investigated. The FeCo lattice stabilised via VN addition were characterised by high Ku magnitudes exceeding 10^6 J・m^-3. The obtained bct structure remained stable even for the films with thicknesses of 100 nm, suggesting its possible use in bulk systems.
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